Congenital heart defects are the most common birth defect in humans, affecting up to 1:100 pregnancies. Children born with congenital heart defects often suffer significant morbidity and mortality. Our understanding of the molecular basis of heart development has increased dramatically in the last 10 years. Proper patterning of the heart results from a complex series of signaling events and tissue interactions. Neural crest cells are critical for both cardiovascular and craniofacial development. Many human syndromes demonstrate combinations of defects in these two areas. Preliminary data generated by our lab suggests that Fgf signaling plays a critical role in neural crest development and alterations of these signals result in craniofacial and cardiac defects. Using recently developed genetic techniques such as the Cre/LoxP system, we propose to study the role of Fgf signaling on neural crest development in the mouse.